Computer systems and other electronic components typically contain one or more integrated circuits (ICs). High-speed ICs that consume a lot of power (on the order of a few Watts or more), such as, for example, processors and chipsets, require that the heat generated by the IC be conducted away from the IC and dissipated. If the heat generated by the IC is not removed, performance of the IC is degraded. In some cases, an IC can become so hot that it will actually destroy itself, causing the computer system to stop functioning. Therefore, it is necessary to design heat removal systems into computer systems that use high-speed ICs.
A heat removal system typically comprises some mechanism to transfer heat from the IC and to dissipate the heat to the ambient environment. Transferring heat from the IC is done using a thermally conductive material, such as copper or aluminum, thermally coupled to the IC. The heat is then dissipated by either increasing the surface area for the heat to dissipate from, increasing air flow (or liquid flow) across a heated surface, or both. Some heat sinks include fins and heat spreader plates that increase heat dissipation surface area. Other heat sinks use a fan to blow air across a surface.
ICs in a computer system are mounted on one or more printed circuit boards (PCBs) that provide a durable substrate base for the ICs. One important feature of a PCB is that the PCB includes electrical interconnects printed onto one or more surfaces of the PCB. These electrical interconnects are coupled to the ICs mounted on the PCB. The electrical interconnects are designed to electrically couple various lCs together in a useful manner on the PCB.
Some PCBs include multiple layers of electrical interconnects, called signal layers. These multiple signal layers increase the degree of integration of the PCB by allowing signals to be routed across electrical interconnects that pass under or over other ICs or electrical interconnects of the PCB rather than around them. For example, a PCB may have signal layers formed on both the top and bottom of the PCB substrate. This effectively doubles the available PCB area for mounting lCs and other components as compared to a PCB that only has a signal layer on one side of the substrate. Signal layers are also formed within the PCB substrate, between the upper and lower PCB surfaces.
Interspersed between these signal layers are ground and power planes, each separated by a dielectric material typically comprising fiberglass. lCs and other components coupled to the PCB tap their ground and power sources from the ground and power planes within the PCB.
Unfortunately, the need to thermally dissipate heat generated by lCs mounted to one side of a PCB can have the effect of limiting the available electrical interconnect space on the opposite side of the PCB. This is because through-holes are made through the PCB to thermally couple an IC mounted on top of a PCB to a heat sink mounted directly beneath the IC on the bottom of the PCB. The through-holes and heat sink prevent electrical interconnects of the signal layer printed on the bottom of the PCB from crossing through this region. In addition, ground planes, power planes, and electrical interconnects of signal layers formed within the PCB, between the upper and lower PCB surfaces, must be routed around the through-holes formed through the PCB. As a result, the size of the PCB increases to accommodate the re-routing of the electrical interconnects. This increases the manufacturing cost, increases the size of the electronic component containing the PCB, and decreases the desirability of the final product to the consumer.